The present disclosure generally relates to systems and methods for detecting flame holding in a gas turbine engine, and more particularly relates to systems and methods for detecting flame holding in the combustor casing of a gas turbine engine.
Many gas turbines include a compressor, a combustor, and a turbine. The compressor creates compressed air, which is supplied to the combustor. The combustor combusts the compressed air with fuel to generate an air-fuel mixture, which is supplied to the turbine. The turbine extracts energy from the air-fuel mixture to drive a load.
In many cases, the gas turbine includes a number of combustors. The combustors may be positioned between the compressor and the turbine. For example, the compressor and the turbine may be aligned along a common axis, and the combustors may be positioned between the compressor and the turbine at an entrance to the turbine, in a circular array about the common axis. In operation, air from the compressor may travel into the turbine through one of the combustors.
The combustors may be operated at a relatively high temperature to ensure the mixture of air and fuel is adequately combusted, improving efficiency. One problem with operating the combustors at a high temperature is that a relatively high level of nitrogen oxides (NOx) may be generated, which may have a negative impact on the environment.
To reduce NOx emissions, many modern gas turbines employ premixing fuel nozzles. For example, each combustor may be supported by a number of fuel nozzles, which may be positioned in a circular array about the combustor. During normal operation, the air from the compressor enters the combustor via the fuel nozzles. Within the fuel nozzles the air is mixed with fuel to form an air-fuel mixture. The air-fuel mixture is then combusted in the combustor. Pre-mixing the air and fuel permits operating the combustors at relatively lower temperatures, which reduces the NOx produced as a by-product of the combustion process.
To achieve further performance advantages, some combustors employ fuel injectors that are positioned upstream of the fuel nozzles. These fuel injectors will collectively be referred to herein as “combustor casing fuel injectors,” and, unless stated otherwise, these are defined to include fuel injectors within the combustion system of a gas turbine engine positioned between the compressor and the fuel nozzles. As stated above, many combustion systems pre-mix fuel and air within fuel nozzles. It will be appreciated that the present invention is aimed at the staged pre-mixing that takes place in some combustors upstream of this.
One such system, for example, is generally referred to as an annular quaternary fuel distributor. As described in more detail below, this type of system injects fuel into the compressed air discharged by the compressor as this flow of air moves toward the fuel nozzles. In certain cases, as described in more detail below, the annular quaternary fuel distributor injects fuel into an annulus passageway that is defined by the combustor casing and the cap assembly. It will be appreciated by one of ordinary skill in the art that pre-mixing fuel in this manner may be employed to mitigate combustor instability, to provide better fuel/air mixing, improve flame holding margin of the downstream fuel nozzles, as well as to reduce NOx emissions.
However, combustor casing fuel injectors present their own problems. For example, the combustor casing fuel injectors may catch fire and/or retain flame, which, as referred to herein, creates a situation of combustor casing flame holding. One common reason for flame holding in the combustor casing is flashback, wherein flame travels backward from the combustion zone of the combustor into the fuel nozzle and then from within the fuel nozzle to within the combustor casing. Another common reason for flame holding in the combustor casing is auto-ignition, wherein the fuel within the combustor casing independently catches fire due. This may occur due to irregularities in the fuel composition, the fuel flow, the air flow, or the fuel nozzle surface, among other reasons. Regardless of the cause, the combustor casing may tend to hold or retain the flame, which may damage the combustor, the fuel nozzles that reside downstream, or other portions of the gas turbine.
So that remedial action may be taken to reduce or eliminate flame holding within the combustor casing, techniques have been developed to detect the presence of flame within this area. However, many of these techniques employ sensors, such as temperature sensors, photon emission sensors, or ion sensors, among others. These types of sensors would have to be positioned at several locations within the combustor casing. More specifically, because of the size and configuration of the combustor casing, these types of sensors would have to be placed at a multitude of locations to ensure that flame is detected at the locations at which it might be held. As one of ordinary skill in the art will appreciate, installing and monitoring a plurality of these types of sensors would be expensive.
Accordingly, there is a need for systems and methods that accurately and efficiently detect the presence of a flame holding in the combustor casing of gas turbine engines.